A suppressor protein which inhibits lymphocyte activation by allogeneic cells of mitogens will be purified from M. Arginini broth cultures and characterized with respect to physical properties, mode of action, kinetics, etc. Further studies on the stimulation of normal human lymphocytes by lymphoblast cell lines (LCL) of B vs T cell origin will also be conducted. Glutaraldehyde fixed LCL and solubilized membrane antigens will be tested. The spontaneous cell mediated cytotoxicity (SCMC) of LCL by normal human lymphocytes will be further investigated and compared to antibody dependent and T-cell mediated cytotoxic mechanisms. Energy requirements, antigen identity, specificity, and the nature of receptors on effector cells will studied.